Molecular triplets and other metastable states for excitonic quantum batteries
Excitonic quantum batteries, based on organic fluorescent molecules embedded in optical microcavities, offer a room-temperature platform for studying collective effects in energy storage and developing applications. Recent experiments have offered evidence of superabsorption, a collective enhancement to the light absorption rate of organic molecules which leads to a scalable power density. However, they have also highlighted the challenge posed by rapid radiative decay of fluorescent molecules, which limits the energy storage lifetime. Current strategies to overcome this trade-off focus on controlling the coupling between the absorbing manifold and that used for energy storage. In this chapter, we review three implementations of this design principle: transferring energy from optically excited states to long-lived dark triplet states, generating triplet pairs and higher-spin states through singlet exciton fission, and forming charge-separated states. We discuss each mechanism from both theoretical and experimental perspectives, with particular emphasis on recent device implementations that have extended storage times by several orders of magnitude. We conclude with a cross-platform outlook on the role of metastable states across coherent and room-temperature implementations, from neutral atom arrays to masers and colour centres.